1. Field of the Invention
The invention relates to signaling networks for telephone systems, and particularly to apparatus and concomitant methods for providing signaling paths within a network of service switching points.
2. Description of the Existing Art
Modern telephony networks contain two networks, one for carrying communication signals (a communications network) and another for carrying signaling and switching control signals (a signaling network). The signaling and switching control signals are hereinafter referred to as signaling messages. These two networks, each containing a plurality of connective pathways interconnecting a plurality of nodes, operate in a symbiotic manner. The signaling network creates a specific path by activating certain switches within a communications network to facilitate formation of a communication path between, for example, a telephone at one station and a telephone at another station. As a result of the communication path, the parties at each station can communicate to one another through the communications network. Alternatively, the signaling network is used to communicate information such as network management and control information between nodes.
Generally, individual central offices form nodes within both the signaling network and the communications network. Each central office contains switching equipment controlled by the signaling messages to produce communication paths through the node. Additionally, each central office contains communication equipment that is used for communication signal processing within the node. However, though the communications network and the signaling network use the same nodes, signaling messages and communication signals are not propagated along the same connective pathways (trunks) between nodes. Since the signaling network operates using signaling messages that do not propagate along the communication network trunks, the signaling network is said to facilitate out-of-band signaling, i.e., the signaling network is an out-of-band network.
Such out-of-band signaling has become standard in the United States telephone system and the telephony equipment industry. The specification for a United States out-of-band signaling network standard is known as the United States National Common Channel Signaling System Number 7 (hereinafter referred to as "SS7"). This standard is specified in American National Standards Institute (ANSI) Standard T1.111-1988. International SS7 networks are specified in Consultative Committee on International Telephone and Telegraph (CCITT) Recommendation Q.705. This recommendation contains a specification for an international Common Channel Signaling (CCS) Network standard. CCS, as with SS7, is a signaling network overlay to a voice carrying trunk circuit network (communications network). SS7 and CCS use almost identical signaling protocols for propagating packet-switched signaling messages through the signaling network. Since the United States protocols and the international protocols are nearly identical, the remainder of this discussion focuses primarily upon SS7.
Typically, an SS7 network comprises a number of SS7 nodes interconnected by various types of pathways. In particular, the SS7 nodes include, inter alia, service switching points (SSPs) and signaling transfer points (STPs). Other types of nodes are used within an SS7 network; however, these nodes are not relevant here and will not be discussed. Therefore, for simplicity, an SS7 network is hereinafter defined as only containing SSPs and STPs.
Generally, the SS7 nodes are arranged within the network in groups. Groups of STPs and SSPs form individual local access transport areas (LATAs). These individual LATAs are interconnected by STPs within a long distance carrier portion of the SS7 network. Typically, individual LATAs are operated by individual Bell Operating Companies (BOCs). However, independent telephone companies (ITCs) can also exist within a given LATA and operate distinct clusters of SSPs (hereinafter referred to as ITC clusters) therein. In some instances, an ITC cluster, rather than being solely contained within a single LATA, may overlap and operate within two geographically adjacent LATAs.
During various signaling message routing operations originating within an ITC cluster (discussed below), an STP operated by the BOC (hereinafter referred to as BOC STPs) must be accessed. Traditionally, each SSP within an ITC cluster was individually connected to one or more BOC STPs. However, since independent telephone companies that operate these SSPs are charged by the BOC for each physical link connected to the BOC STP (known as an A-link), the independent telephone companies have begun to use A-link consolidators (ALC) to limit the number of connections to each BOC STP. An ALC is an SSP that is specially designed to route signaling messages from a number of A-links to a single A-link. In general, a number of SSPs forming a particular SSP cluster is connected to one or more ALCs. Each ALC, in turn, is connected through two A-links to one or more BOC STPs. As such, the independent telephone companies which operate a given SSP cluster are only charged for two physical links from each ALC to the BOC STPs.
As is well known in the art, certain messages propagating from any one SSP to another SSP, even if the two SSPs are within the same LATA or even within the same cluster, must access a BOC STP. One type of these signaling messages is known as custom local area signaling services (CLASS) messages. CLASS messages are sent between SSPs to facilitate use of special customer services such as automatic recall (AR), automatic callback (AC) and screening list editing (SLE). Those skilled in the art will readily understand the operation and function of these services as well as the utilization of CLASS messages to facilitate these services. Therefore, CLASS messages and their function will not be discussed in detail herein.
To appropriately route a CLASS message, the SS7 protocol requires a target destination number, e.g., a telephone number dialed by a caller, to be translated into a destination point code. The destination point code typically is a 24-bit address of a node (switching or signaling point) within the SS7 network. The node indicated by the destination point code is typically the node connected to the telephone associated with the target-destination number. The translation process is accomplished by a BOC STP within the LATA containing the ITC cluster from which the call is made. An A-link connects the STP to an ALC within the ITC cluster. In operation, each CLASS message that must be transferred between SSPs is routed to an STP for translation. As a result of the translation, the STP re-addresses the CLASS message and sends the message either through the SS7 network to a destination SSP within another SSP cluster (inter-cluster signaling) or to a destination SSP within the cluster from which the message originated (intra-cluster signaling). In either instance, the appropriately re-addressed message is received by the destination SSP. In response to the signaling message, the destination SSP performs appropriate switching functions or, depending on the type of CLASS message, the SSP returns certain information, e.g., a previously dialed telephone number, to the SSP which sent the message (originating SSP).
Unfortunately, use of an STP engenders several drawbacks. First, if the independent telephone companies could avoid STP access for destination number translation, then the delay inherent is routing a message between SSPs via an STP, could be greatly reduced. Furthermore, STP network access adds an unnecessary time delay and network complexity to intra-cluster message routing caused by accessing the STP and subsequently returning a translated message to the cluster that accessed the STP. Now, if the SS7 functions, in particular the translation function, are performed within an SSP itself for intra-cluster signaling message routing, a substantial reduction in network complexity and processing time savings could be achieved.
Thus, a need exists in the art for a technique that performs some of the SS7 network functions within the SSPs of an SSP cluster. In particular, a need exists for apparatus and concomitant methods for translating destination numbers into destination point codes within the SSPs of an SSP cluster.